142 Meat refrigeration tunnels are of simpler construction but are restricted by the length of belt necessary to achieve the cooling time required and on the space available in most factories. Spiral freezers are therefore a more viable alter native 7.3 Contact freezers Contact freezing methods are based on heat transfer by contact between products and metal surfaces, which in turn are cooled by either primary or secondary refrigerants. Contact freezing offers several advantages over air cooling, i.e. there is much better heat transfer and significant energy savings However, the need for regularly shaped products with large flat surfaces is a major hindrance Modern plate cooling systems differ little in principle from the first contact freezer patented in 1929 by Clarence Birdseye. Essentially the refrigerant(Fig. 7. 2). A hydraulic cylinder is used to bring the freezing plates into pressure contact with the product. These plates can be either horizontal or vertical Good heat transfer is dependent on product thickness, good contact and the conductivity of the product. Plate freezers are often limited to a maximum thickness of 50-70mm Good contact is a prime requirement. Air paces in packaging and fouling of the plates can have a significant effect on cooling time, for example a water droplet frozen on the plate can lengthen the freezing time in the concerned tray by as much as 30-60% General advantages of plate freezers over air-blast carton fre Hydraulic ram Product Fig. 7.2. Example of a horizontal plate freezertunnels are of simpler construction but are restricted by the length of belt necessary to achieve the cooling time required and on the space available in most factories. Spiral freezers are therefore a more viable alternative. 7.3 Contact freezers Contact freezing methods are based on heat transfer by contact between products and metal surfaces, which in turn are cooled by either primary or secondary refrigerants. Contact freezing offers several advantages over air cooling, i.e. there is much better heat transfer and significant energy savings. However, the need for regularly shaped products with large flat surfaces is a major hindrance. Modern plate cooling systems differ little in principle from the first contact freezer patented in 1929 by Clarence Birdseye. Essentially the product is pressed between hollow metal plates containing a circulating refrigerant (Fig. 7.2). A hydraulic cylinder is used to bring the freezing plates into pressure contact with the product. These plates can be either horizontal or vertical. Good heat transfer is dependent on product thickness, good contact and the conductivity of the product. Plate freezers are often limited to a maximum thickness of 50–70 mm. Good contact is a prime requirement.Air spaces in packaging and fouling of the plates can have a significant effect on cooling time, for example a water droplet frozen on the plate can lengthen the freezing time in the concerned tray by as much as 30–60%. General advantages of plate freezers over air-blast carton freezers include: 142 Meat refrigeration Hydraulic ram Freezing plate Product Separated plates Closed plates Fig. 7.2. Example of a horizontal plate freezer